linux-kernel - [PATCH] kernel/sys.c : Get rid of expensive divides in groups

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Message-ID: <47686269.2090103@cosmosbay.com>
Date:	Wed, 19 Dec 2007 01:14:33 +0100
From:	Eric Dumazet <dada1@...mosbay.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
CC:	Linux kernel <linux-kernel@...r.kernel.org>
Subject: [PATCH] kernel/sys.c : Get rid of expensive divides in groups_sort()

groups_sort() can be quite long if user loads a large gid table.

This is because GROUP_AT(group_info, some_integer) uses an integer divide.
So having to do XXX thousand divides during one syscall can lead to very high 
latencies. (NGROUPS_MAX=65536)

In the past (25 Mar 2006), an analog problem was found in groups_search()
(commit d74beb9f33a5f16d2965f11b275e401f225c949d ) and at that time I changed 
some variables to unsigned int.

I believe that a more generic fix is to make sure NGROUPS_PER_BLOCK is unsigned.

Signed-off-by: Eric Dumazet <dada1@...mosbay.com>

View attachment "NGROUPS_PER_BLOCK.patch" of type "text/plain" (1272 bytes)